Watering Urban Vegetated Patches: Computing Ecologically Useful Runoff Subsidy

English Abstract:

The impact of impervious surfaces on urban storm and base flows is well established and watershed scale models often include percent impervious cover as a key parameter. For some research and management questions, however, a more spatially explicit characterization of how urban surfaces redirect incoming precipitation is required. In situations where water availability limits vegetation productivity, additional water inputs to vegetated patches from adjacent impervious surfaces may be hydrologically significant. To estimate these potentially ecologically useful (or “green water”) contributions from impervious surfaces requires differentiating between connected and disconnected impervious surfaces. Connected impervious surfaces route surface runoff directly into storm sewers or waterways. Disconnected impervious surfaces route water to adjacent areas. How much of the runoff contributed from disconnected impervious surfaces that is ecologically useful also depends on climate, soil, and vegetation characteristics, and the interactions between them. Recent advances in eco-hydrologic models, such as RHESSys (Regional Hydro-Ecologic Simulation System), provide tools for quantifying these interactions and can aid urban planners in assessing urban design impacts on vegetation. We present results from an application of RHESSys to a semi-urban watershed in Santa Barbara, California, and show how disconnected impervious surfaces could potentially supplement urban irrigation requirements; but only if urban vegetation is a deeply rooted species such as native chaparral. We note parallels between these findings and results from eco-hydrologic analysis of vegetated patches in natural semi-arid systems. We also contrast these results with estimates from a more humid urban setting in Charlotte, North Carolina. Our results provide a basis for future model development and application, and demonstrate the utility of emerging tools that facilitate model-data integration in eco-hydrology.